Perhaps (I emphasize "perhaps") he was sure:

Geary was set up in the whorehouse while Michael was visiting Roth and Pentangeli--that is, within days of the "oily hair" meeting. Now, Michael left Tom in charge, and Tom obviously set the brothel murder/blackmail plan in motion. But I doubt Tom would have ordered the murder, and the blackmail of someone as important as Geary, without Michael's prior approval.

I'm guessing that, since Geary was a regular at Fredo's brothel and in that girl's bed ("...we done it be-FOE-wer..."), Fredo must have spotted his distinguished client during earlier visits and let Michael know about it ("Hey, Mikey, guess who's a regular at my joint? Yer pal Senator Geary. Yeah, he's into some real kinky s**t with one o'my girls..."). Michael would have stashed that precious bit of knowledge in his mental vault. So, when Geary muscled him, that calm look and tone ("Oh Senator, you can have my answer now...") came from his mental vault: "I know your weakness and I'm gonna get you." After Geary left, he probably told Tom and Neri, "Tell Fredo to let me know the next time Geary shows up in the brothel. We'll nail his ass to the wall." He may even have reminded Tom about that plan before he left following the shooting.

True, but I just think he was taking a chance that Geary could be set up. In fact, he could have agreed to Geary's terms at the meeting and still set him up.

I think that Michael saw Geary as an "easy fix"... Geary acting all high and mighty the way he did... after all... Michael took care of ALL family business while in church. Geary was never a challenge to Michael. He saw him and stupid and vulnerable.

is always good to remember a great thread like that. I see someone already did it less than an year ago but what the hell....

johnny ola (the BB MEMBER, not the CHARACTER!) already mentioned the scene in GF1 when Sonny screws it up by "letting someone outside the family (Sollozo) know what he's thinking". if Sonny just kept his mouth shut that day... anyway, there is no way that Michael would give Tom a "didn't you do your job?" look in front of Geary. I agree that Michael assumed it was all set up and Geary creating one more problem was a big surprise, that's why he and Tom exchanged looks.